Cooling structure of piston in engine

ABSTRACT

A cooling structure of a piston in an engine, in which oil is injected toward a top portion of the piston reciprocating within a cylinder of the engine to cool the top portion of the engine. The cooling structure may include an oil guide surface including at least a thick wall portion of a peripheral wall of the piston which is provided with a ring groove and extends continuously from an inner surface of the peripheral wall of the piston to a reverse surface of the top portion of the piston.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to a structure for cooling a piston by injecting oil to the piston in an engine mounted in a vehicle such as a motorcycle. More particularly, the present invention relates to a structure of a reverse surface of the piston to which the oil is injected.

2. Description of the Related Art

Conventionally, in a high-power reciprocating engine, oil injected to a piston reciprocating within a cylinder from below (bottom dead center side), to directly inject the oil to a reverse, i.e., underside, surface of a top portion of the piston which is exposed to a high-temperature combustion gas, thereby cooling the piston. For example, in an engine disclosed in Japanese Laid-Open Patent Application Publication No. 2007-231787, as shown in FIG. 10 in this Publication, an oil injection pipe is provided in the vicinity of a lower end of a peripheral surface of the cylinder, and the oil is injected toward a piston located above the oil injection pipe through an oil injection hole (oil jet) provided in the oil injection pipe.

As shown in FIG. 10 of this Publication, an oil injection direction is inclined (oblique) with respect to a cylinder axis. The injected oil travels in an upward direction radially inward of the cylinder, and is directly applied to a reverse surface of a top portion of the piston near a bottom dead center of a reciprocating stroke of the piston.

Although in the conventional engine, the oil is injected to the piston when it is near the bottom dead center of the stroke, the oil is not injected to the piston in an upper location of the stroke. The oil injected to the piston when it is near the bottom dead center flows downward due to an inertia force as the piston moves upward. Therefore, the oil is not sufficiently fed to the reverse surface of the top portion of the piston which is high in heat load in a state in which the piston is near a top dead center of the stroke. It is presumed that a cooling efficiency of the piston is not high such a situation.

If the oil is injected to the piston in parallel with a cylinder axis from below the piston, then the oil can be injected to the piston which is near the top dead center of the stroke. In this case, because of a limited position of the oil injection hole, the oil injected toward the piston does not smoothly reach the reverse surface of the top portion of the piston which is high in heat load.

To prevent the injected oil from contacting a rotating crankshaft or a connecting rod, the oil injection hole is inevitably positioned in the vicinity of a periphery of a cylinder bore. Therefore, the oil injected in parallel with the cylinder axis through the oil injection hole travels through a region near an inner surface of a peripheral wall of the piston.

However, typically, a groove into which a piston ring is fitted is formed on an upper portion of the peripheral wall of the piston, and correspondingly, the upper portion of the peripheral wall has a greater wall thickness. Therefore, a stepped portion is formed between the upper portion with a greater wall thickness and a lower portion of the peripheral wall with a relatively small wall thickness. In this structure, the oil traveling through the region in the vicinity of the inner surface of the peripheral wall of the piston collides with the stepped portion and scatters. As a result, the oil is less likely to reach the reverse surface of the top portion of the piston which is high in heat load.

SUMMARY OF THE INVENTION

The present invention addresses the above described condition, and an object of the present invention is to cause oil injected toward a top portion of a piston from below to smoothly reach a reverse surface of the top portion of the piston, thereby providing a higher cooling efficiency.

To solve the above described problem, according to an aspect of the present invention, there is provided a cooling structure of a piston in an engine, in which an oil is injected toward a top portion of the piston reciprocating within a cylinder of the engine to cool the top portion of the engine, comprising: an oil guide surface including at least a thick wall portion of a peripheral wall of the piston which is provided with a ring groove and extends continuously from an inner surface of the peripheral wall of the piston to a reverse surface of the top portion of the piston.

As used herein, an upper side and a lower side mean a top dead center side and a bottom dead center side, respectively, of a stroke of a reciprocation of the piston, in a direction of an axis of a cylinder in which the piston reciprocates, and a direction defined by the upper side and the lower side does not always conform to a vertical direction depending on how the engine is mounted in a vehicle. Preferably, the oil guide surface is smoothly continuous from the inner surface of the peripheral wall of the piston to the reverse surface of the top portion of the piston. Alternatively, the oil guide surface may have a curvature which changes greatly or some unevenness, so long as the oil guide surface has a recessed shape as a whole from the peripheral wall of the piston to the top portion of the piston, in a cross-section including a center line of the piston.

In this configuration, the oil injected to the oil guide surface of the piston from below travels smoothly along the oil guide surface and reaches the reverse surface of the top portion of the piston, thereby efficiently cooling the top portion of the piston which is high in heat load. The oil injected to the oil guide surface travels up and down on the oil guide surface by an inertial force of the piston reciprocating. However, an inertia force for directing the oil upward is exerted in a state in which the piston is near the top dead center of the stroke. Therefore, the oil is caused to concentrate on the top portion of the piston which is high in heat load and cools it effectively.

Protruding portions may be provided to sandwich the oil guide surface from both sides in a circumferential direction of the piston. In this structure, the oil injected to the oil guide surface is less likely to spread in the circumferential direction of the piston, which allows the oil to easily reach the reverse surface of the top portion of the piston.

The thick wall portion of the peripheral wall of the piston may have a recess at an inner side thereof, and a bottom surface of the recess may constitute a portion of the oil guide surface. Or, a swelling portion protruding inward may be formed continuously with the thick wall portion of the peripheral wall of the piston, and the oil guide surface may be formed to extend from the swelling portion to the thick wall portion. Or, the recess and the swelling portion may be combined to form the oil guide surface from the swelling portion to the bottom surface of the recess.

The oil guide surface may be formed at an exhaust side of the piston. The exhaust side of the piston is a side closer to an exhaust port in a case where the piston fittingly inserted into the cylinder is equally divided into a side closer to an intake port and a side closer to the exhaust port when viewed in a direction of the cylinder axis. A temperature rises more easily in the cylinder and the exhaust side of the piston than in the intake side. Therefore, it is important to inject the oil to the exhaust side of the piston to cool it.

The cooling structure of the piston in the engine may comprise: an injection nozzle for injecting the oil to the piston, the injection nozzle being directly coupled to a main gallery of the oil inside of a crankcase. The oil discharged from the oil pump is distributed to a bearing of a crankshaft, a valve driving system of the engine, and a transmission through oil passages branching from the main gallery. By coupling the injection nozzle to the main gallery which is not fed with the oil yet, an oil injection pressure increases, which cools the piston more efficiently.

The main gallery is typically disposed below a journal member to which the crankshaft is pivotally mounted within the crankcase. Therefore, if the oil is injected from the injection nozzle provided in the main gallery, the oil can be injected to the piston at a timing at which the oil will not contact the rotating crankshaft or the connecting rod, i.e., for a period corresponding to only a portion of the stroke of the piston.

In view of the above, the injection nozzle may be positioned to inject the oil to the piston from a timing corresponding to a location below a center of a stroke from a bottom dead center to a top dead center, when the piston is moving upward from the bottom dead center toward the top dead center. A speed of the piston moving upward decreases gradually from the location near the center of the stroke from the bottom dead center to the top dead center. Therefore, the oil injected to the oil guide surface from the timing corresponding to the location near the center of the stroke flows toward the top portion of the piston by an upward inertia force.

The oil guide surface may be recessed relative to a portion of the peripheral wall of the piston which portion is adjacent to the oil guide surface in a circumferential direction of the piston. This makes it possible to prevent a stiffness of the piston from being reduced undesirably.

According to another aspect of the present invention, there is provided a cooling structure of a piston in an engine, in which oil is injected toward a top portion of the piston reciprocating within a cylinder of the engine to cool the top portion of the engine, comprising: an oil guide surface extending continuously from an inner surface of a peripheral wall of the piston to a reverse surface of the top portion of the piston, the oil guide surface being recessed relative to a portion of the peripheral wall of the piston which portion is adjacent to the oil guide surface in a circumferential direction of the piston.

In this configuration, the oil injected to the oil guide surface of the piston from below travels smoothly along the oil guide surface and reaches the reverse surface of the top portion of the piston, thereby efficiently cooling the top portion of the piston which is high in heat load. The oil injected to the oil guide surface travels up and down on the oil guide surface by an inertial force of the reciprocating piston. However, an inertia force for directling the oil upward is exerted in a state in which the piston is near the top dead center of the stroke. Therefore, the oil is caused to concentrate on the top portion of the piston which is high in heat load and cools it effectively.

An injection direction of the oil may be substantially parallel to an axis of the piston, and the oil is injected from an opposite side of the piston with respect to a crankshaft of the engine. This makes it possible to easily set the injection position of the oil.

In a state in which the piston is at top dead center, the oil guide surface may be positioned so as not to overlap with a connecting rod of the engine when viewed from a direction of the axis of the piston. This makes it possible to guide the oil to the oil guide surface without contacting the connecting rod.

The oil guide surface may be positioned so as not to overlap with the crankshaft of the engine when viewed from a direction of the axis of the piston. This makes it possible to guide the oil to the oil guide surface without contacting the crankshaft.

An injection position of the oil may be set so as not to overlap with the crankshaft of the engine when viewed from the direction of the axis of the piston. This makes it possible to guide the oil to the oil guide surface without contacting the crankshaft.

The above and further objects, features and advantages of the invention will more fully be apparent from the following detailed description with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a left side view of an engine according to an embodiment of the present invention.

FIG. 2 is a cross-sectional view showing a piston, a connecting rod, a crankshaft, and others within a cylinder of the engine.

FIG. 3 is a perspective view of a crankcase showing a schematic configuration of an oil passage.

FIG. 4 is a cross-sectional view showing an oil guide surface of the piston.

FIG. 5 is a perspective view showing the oil guide surface when the piston is seen from below.

FIG. 6 is a view showing oil injected to the piston near a bottom dead center, corresponding to FIG. 2.

FIG. 7 is a view showing a state in which the piston is moving upward, corresponding to FIG. 6.

FIG. 8 is a view showing a state in which the piston is near a top dead center, corresponding to FIG. 6.

FIG. 9A is a view showing another embodiment of the oil guide surface, corresponding to FIG. 4.

FIG. 9B is a view showing another embodiment of the oil guide surface, corresponding to FIG. 4.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an engine E according to the present embodiment of the present invention will be described with reference to the drawings. The engine E 1 is mounted in, for example, a motorcycle (not shown). Hereinafter, a rightward and leftward direction is from a perspective of a rider straddling the motorcycle in which the engine E is mounted.

[Overall Configuration of Engine]

FIG. 1 is a left side view showing a schematic configuration of the engine E according to an embodiment of the present invention. FIG. 2 is a cross-sectional view showing a piston, a connecting rod, a crankshaft, and others within a cylinder of the engine E. In FIGS. 1 and 2, an air-intake system, an exhaust system, etc., are detached, and an engine body is mainly shown. For example, the engine E is an in-line two-cylinder gasoline engine in which two cylinders C (shown in FIG. 2) are arranged in the rightward and leftward direction. A cylinder head 2 is attached to an upper portion of a cylinder block 1 provided with the cylinders C and closes upper ends of the cylinders C. A piston 3 (shown in FIG. 2) is reciprocatingly and fittingly inserted into each of the cylinders C1. A combustion chamber is formed above a top portion of the piston 3.

As indicated by a broken line in FIG. 1, in the cylinder head 2, an intake port 20 and an exhaust port 21 are formed for each of the cylinders C and open in a ceiling portion of the combustion chamber. An opening of the intake port 20 and an opening of the exhaust port 21 which face an interior of the cylinder C are opened and closed by an intake valve and an exhaust valve, respectively, which are actuated by an intake shaft and an exhaust camshaft (not shown). For example, in the present embodiment, a DOHC valve driving mechanism including two camshafts, which are the intake camshaft and the exhaust camshaft, is provided, and a head cover 4 covers the cylinder head 2 from above.

An ignition plug 22 is disposed in the cylinder head 2 for each cylinder C and faces the combustion chamber, being formed through its ceiling portion substantially at a center of the ceiling portion. An ignition circuit 23 is coupled to an upper portion of the ignition plug 22. An upper portion of the ignition circuit 23 penetrates the head cover 4. The ignition circuit 23 supplies a current to the ignition plug 22 at a predetermined ignition timing for each cylinder C, to ignite and combust an air-fuel mixture. The combustion causes the piston 3 to be pushed down, and a rotational force is transmitted to a crankshaft 26 via the connecting rod 25.

A shaft portion 26 a of a crankshaft 26 is located on a downwardly extended line of a cylinder axis X. A larger-end portion 25 a of the connecting rod 25 is slidably mounted to a crankpin 26 b which is eccentric. A smaller-end portion 25 b of the connecting rod 25 is slidably mounted to a piston pin 35. When the piston 3 reciprocates in a direction of the cylinder axis X, the crankshaft 26 rotates in a counterclockwise direction as indicated by an arrow of FIG. 2.

As shown in FIG. 1, the intake port 20 for feeding the air-fuel mixture to the combustion chamber inside of the cylinder C extends obliquely upward from the ceiling portion of the combustion chamber and opens in a rear surface of the cylinder head 2. In a state in which the engine E is mounted in the motorcycle, the cylinder block 1 and the cylinder head 2 are slightly inclined forward. Two throttle bodies (not shown) are provided on the rear surface of the cylinder head 2 such that the throttle bodies are coupled to the intake ports 20, respectively, which are arranged side by side in the rightward and leftward direction.

The exhaust ports 21 for exhausting a combustion gas from the combustion chambers of the cylinders C, respectively, open on a front surface of the cylinder head 2 such that they are arranged side by side in the rightward and leftward direction. Exhaust manifolds are coupled to the exhaust ports 21, respectively. Although not shown, two exhaust pipes of the exhaust manifolds extend downward in a region forward relative to the engine E, then are curved in a rearward direction, and then are merged into a single exhaust pipe below the engine E. The single exhaust pipe is coupled to a catalyst, an exhaust muffler, etc.

FIG. 3 is a perspective view showing the crankcase 5 with the cylinder block 1, the cylinder head 2 and the crankcase 5 detached. The crankcase 5 is, for example, a component cast using aluminum alloy. The crankcase 5 includes an upper crankcase 50 mounted to the cylinder block 1 and a lower crankcase 51 mounted to a lower portion of the upper crankcase 50. A journal bearing (not shown) is mounted to a joint surface at which the upper crankcase 50 and the lower crankcase 51 are joined together and supports the crankshaft 26 such that the crankshaft 26 is rotatable.

Although in the present embodiment, a transmission case 5 a and a balancer case 5 b accommodating a moment balancer 53 are integral with the rear portion of the crankcase 5, the present invention is not limited to this. In the present embodiment, for example, a mesh gear transmission (not shown) is accommodated into the transmission case 5 a. An output shaft 54 of the transmission protrudes at a left side of the transmission case 5 a. A water pump 55 is disposed at a left side of the transmission case 5 a. The water pump 55 feeds cooling water introduced through a pipe member 56 from a radiator (not shown in FIG. 3) to the cylinder block 1 through a pipe member 57.

As shown in FIG. 1, an oil pan 6 for reserving a lubricating oil in a lower portion of the crankcase 5 (and transmission case 5 a). The oil pan 6 has a shape in which a deep bottom portion is formed in a rear half portion thereof and its depth decreases gradually in a forward direction. A cylindrical oil filter 64 for filtering the oil protrudes forward from a front portion of the crankcase 5.

[Structure of Oil Passage]

The engine E of the present embodiment is configured in such a manner that a driving power is taken out from a driving power transmission path from the crankshaft 26 to the transmission to actuate the oil pump 62 (indicated by broken line in FIG. 3), and the oil pump 62 suctions up the oil from the oil pan 6, and feeds the oil to various lubrication components and members of the engine E, such as the crankshaft 26 and the valve driving mechanism. In the perspective view of the crankcase 5 of FIG. 3, a schematic configuration of an oil passage from the oil pan 6 to a main gallery 66 via the oil pump 62 is depicted by a virtual bold line.

A straightener 60 (indicated by a virtual line in FIG. 3) is disposed in the deep bottom portion of the rear portion of the oil pan 6 and immersed in the oil reserved in the oil pan 6. A first oil passage 61 extends upward from the straightener 60. An upper end of the first oil passage 61 is coupled to the oil pump 62. A second oil passage 63 is coupled to a discharge port of the oil pump 62, then extends downward, then is bent forward, then extends substantially horizontally in a forward direction, and then opens in a front surface of the lower crankcase 51.

A front end of the second oil passage 63 is coupled to an inlet of the oil filter 64. A third oil passage 65 extends in a forward and rearward direction above the second oil passage 63 such that the third oil passage 65 extends substantially in parallel with the second oil passage 63, and is coupled to an outlet of the oil filter 64. A rear end of the third oil passage 65 communicates with the main gallery 66 extending in the rightward and leftward direction and having a greater diameter. In this configuration, upon the engine E running, the oil pump 62 is actuated. The oil is suctioned up from the oil pan 6 to the oil pump 62 through the first oil passage 61, then fed to the oil filter 64 through the second oil passage 63 to be filtered in the oil filter 64, and then fed to the main gallery 66 through the third oil passage 65.

In the present embodiment, the main gallery 66 extends at a lower portion of the crankcase 5, in the rightward and leftward direction such that the main gallery 66 extends substantially in parallel with the center axis of the crankshaft 26. As will be described later, a plurality of oil passages branch at the main gallery 66 to feed the oil to the lubrication components in the engine E. More specifically, the oil is fed to, for example, the journal member of the crankshaft 26 and sliding portions of the crankshaft 26 and of the connecting rod 25, through the oil passages branching from specified locations of the main gallery 66.

As shown in FIG. 3, two pipe members 68 and 69 are coupled to the left end of the main gallery 66 via a joint 67. The first pipe 68 extends rearward from the joint 67, while the second pipe 69 extends forward from the joint 67. The oil is fed to the transmission via the first pipe member 68 and to a starter mechanism (not shown) via the second pipe member 69. In addition, a third pipe member 70 extends to an upper portion of the cylinder head 2 through a space forward relative to the cylinder block 1 (not shown in FIG. 3). A portion of the oil fed to the starter mechanism, etc., as described above is fed to the valve driving mechanism and others of the cylinder head 2 via the third pipe member 70.

[Cooling Structure of Piston using Oil Jet]

In the present embodiment, the main gallery 66 which distributes and feeds the oil to the lubrication components as described above is provided with an oil jet 71 (injection nozzle) for injecting the oil from below toward the piston 3 for each cylinder C. Two oil jets 71 are provided in two locations of the main gallery 66 so as to correspond to exhaust side end portions for the respective cylinders C, although one oil jet 71 is shown in FIG. 2. The oil jets 71 inject the oil as schematically indicated by arrows OJ. By coupling the oil jets 71 to the main gallery 66, an injection pressure of the oil can be increased, which can effectively enhance a cooling efficiency of the piston 3.

When seen from a direction of a center axis of the crankshaft 26 as shown in FIG. 2, and FIGS. 6 to 8, the main gallery 66 is located substantially below the center axis of the crankshaft 26. Each oil jet 71 injects the oil toward the top portion of the piston 3 located thereabove substantially in parallel with a cylinder axis X. Each oil jet 71 is located outside of trajectories drawn by a motion of the rotating crankshaft 26 and a motion of the connecting rod 25. Each oil jet 71 is positioned as far distant as possible from the cylinder axis X as corresponding to the exhaust side (left side) end portion of the cylinder C to ensure a period during which the injected oil does not contact the crankshaft 26 and the connecting rod 25.

In other words, when viewed from the direction of the cylinder axis X, the oil jet 71 is positioned near an exhaust side peripheral portion inside a cylinder bore. The oil injected from the oil jet 71 substantially in parallel with the cylinder axis X travels through a region near an inner surface of a peripheral wall of the piston 3. In a conventional general piston, it is difficult for the oil traveling through the region near the inner surface of the peripheral wall of the piston 3 to reach the top portion of the piston 3 which is high in heat load.

With reference to FIGS. 4 and 5 each showing the piston 3 of the present embodiment as a single form, the piston 3 of a gasoline engine is typically required to be thinned for the purpose of a reduced weight. A thick wall portion is formed in an upper portion 30 a of a peripheral wall 30 provided with a ring groove 3 a into which a piston ring (not shown) is fitted. A stepped portion 30 c is formed between the upper portion 30 a and the following peripheral wall lower portion 30 b which is thinner than the upper portion 30 a. The oil which is going to travel through the region near the inner surface of the peripheral wall 30 of the piston 3 collides with the stepped portion 30 c and scatters. Thus, it is difficult for the oil to reach the top portion 31 of the piston 3.

As shown in FIGS. 4 and 5 in the present embodiment, the piston 3 has a shape in which a lower portion of the thick wall portion at the exhaust side end portion of the peripheral wall upper portion 30 a is recessed to form the groove 32 extending vertically, and a groove bottom surface 32 a is vertically curved gently and smoothly connected to an inner surface of the peripheral wall lower portion 30 b at a lower side and to the reverse surface of the top portion 31 of the piston 3 located at an upper side.

More specifically, a center portion of the reverse surface of the top portion 31 of the piston 3 has a substantially flat surface portion 31 a, and an inclined surface 31 b is formed to surround the flat surface portion 31 a such that its thickness increases toward its outer periphery. An outer peripheral edge of the inclined surface 3 lb is connected to an upper edge of the groove bottom surface 32 a to form a recessed oil guide surface which is continuous from the inner surface of the peripheral wall 30 to the reverse surface of the top portion 31 (i.e., the oil guide surface does not have a portion protruding inward of the piston 3 from the inner surface of the peripheral wall 30 to the groove bottom surface 32 a and to the reverse surface of the top portion 31 in the cross-section of FIG. 4).

As indicated by the arrow OJ in FIGS. 6 to 8, the oil injected to the groove bottom surface 32 a from below substantially in parallel with the cylinder axis X travels upward along the groove bottom surface 32 a and the inclined surface 31 b at an upper side of the groove bottom surface 32 a, i.e., the oil guide surface, and smoothly reaches the reverse surface of the top portion 31 of the piston 3 which is high in heat load.

The groove bottom surface 32 a to which the oil is injected is a bottom surface of the groove 32, and side surfaces 32 b of the groove 32 are provided to sandwich the groove bottom surface 32 a from both sides in a circumferential direction. The oil injected to the groove bottom surface 32 a is less likely to spread in the circumferential direction of the piston 3, which allows the oil to reach the reverse surface of the top portion 31 of the piston 3. In this respect, the side surfaces 32 b of the groove 32 serve as protruding portions sandwiching the oil guide surface from both sides in the circumferential direction.

As can be seen from the left side of FIG. 2, the crankshaft 26 of the engine E rotates in the counterclockwise direction. During a period for which the piston 3 moves downward from a top dead center and then reaches a location slightly above a bottom dead center (e.g., BBDC 5 degrees CA), the connecting rod 25 is located above the oil jet 71 and contacts the oil injected from the oil jet 71. Therefore, during this period, a greater portion of the injected oil collides with the larger end portion 25 a of the connecting rod 25 and scatters, so that the oil is not injected to the piston 3 located thereabove.

By comparison, as shown in FIGS. 6 to 8, during a period for which the piston 3 is moving upward from a location near the bottom dead center to a location near the top dead center, the oil is injected to the piston 3 located thereabove without contacting the connecting rod 25 and the crankshaft 26. In other words, the oil jet 71 is configured to, at the latest, inject the oil to the piston 3 from a timing corresponding to a location below a center of a stroke from the dead bottom center to the top dead center, when the piston 3 is moving upward from the dead bottom center to the top dead center.

[Advantages]

When the engine E of the present embodiment is running, the oil pump 62 is actuated by the rotation of the crankshaft 26, and the oil suctioned up from the oil pan 6 is fed to the main gallery 66 through the oil filter 64. The oil is fed from the main gallery 66 to the lubrication components of the engine E and to the transmission behind the engine E.

A portion of the oil in the main gallery 66 is injected with a pressure from the oil jet 71 for each cylinder C toward the piston 3 located thereabove. As described with reference to FIGS. 6 to 8, during the period for which the piston 3 is moving upward from a location near the bottom dead center to a location near the top dead center, the injected oil is applied to the lower portion of the oil guide surface of the piston 3, i.e., the groove bottom surface 32 a of the groove 32 formed on the peripheral wall 30 of the piston 3. Since the side surfaces 32 b of the groove 32 are provided at both sides of the groove bottom surface 32 a, the oil is less likely to spread in the circumferential direction of the piston 3.

A speed of the piston 3 moving upward decreases gradually from the location near the center of the stroke as shown in FIG. 7. Therefore, the oil injected to the groove bottom surface 32 a flows to the inclined surface 31 b of the top portion 31 of the piston 3 and further to the flat surface portion 31 a at an inner side, by an upward inertia force. That is, the oil injected to the piston 3 smoothly flows along the oil guide surface and reaches the reverse surface of the top portion 31 of the piston 3. As shown in FIG. 8, the oil is injected to the groove bottom surface 32 a until the piston 3 reaches the location near the top dead center (in the illustrated example, ATDC 6 degrees CA).

When the piston 3 reaches the top dead center, the piston 3 starts to move downward. Thereafter, until the piston 3 reaches the location near the center of the stroke, it moves downward with a downward acceleration, as shown in FIG. 2. Therefore, the oil on the oil guide surface (groove bottom surface 32 a and inclined surface 31 b) travels toward the top portion 31 of the piston 3, by an upward inertial force. That is, the oil for cooling the piston 3 is fed to the top portion 31 of the piston 3 which is high in heat load, thereby cooling the piston 3 effectively, during a period for which the air-fuel mixture in the cylinder C is ignited and combusted, within a period from an end of a compression stroke of the cylinder C to a start of an expansion (explosion) stroke of the cylinder C.

In other words, in the present embodiment, the piston 3 has the oil guide surface for allowing the oil injected from the oil jet 71 to be guided to the top portion 31 of the piston 3, and the oil jet 71 is positioned to inject the oil at a suitable timing so that the injected oil is fed to the top portion 31 of the piston 3 effectively, in view of an inertia force of the piston 3.

In the present embodiment, since only the groove 32 is formed on the peripheral wall upper portion 30 a (thick wall portion) of the piston 3, manufacturing cost will not substantially increase as compared to a conventional cooling structure of a piston. In addition, since the groove 32, i.e., the oil guide surface, is formed at the exhaust side of the piston 3, the exhaust side of the piston 3 which is higher in heat load than the intake side of the piston 3 can be cooled by the oil effectively, in addition to the top portion 31 of the piston 3.

Other Embodiment

The cooling structure of the piston of the present invention is not limited to the present embodiment, and can be changed, added or deleted without changing a scope of the invention. For example, instead of the exhaust side of the piston 3, the groove 32 may be formed so as to form the oil guide surface on the intake side of the piston 3, or may be formed in a portion of the piston 3 between the exhaust side and the intake side.

A specific structure of the oil guide surface is not limited to the above embodiment. Although in the present embodiment, the groove 32 is formed on the thick wall portion of the peripheral wall upper portion 30 a of the piston 3, and the oil guide surface is formed by the groove bottom surface 32 a and the inclined surface 31 b continuous with the groove bottom surface 32 a at the upper side, the present invention is not limited to this.

For example, as shown in FIG. 9A, a swelling portion 33 which swells inward of the piston 3 is formed to extend from the peripheral wall upper portion 30 a of the piston 3 to the peripheral wall lower portion 30 b of the piston 3, and an oil guide surface 33 a may be formed on a surface of the swelling portion 33. As shown in FIG. 9A, in a cross-section including a center line of the piston 3, the oil guide surface has a recessed shape (the oil guide surface does not have a portion protruding inward of the piston 3) which is continuous from the inner surface of the peripheral wall 30 of the piston 3 to the reverse surface of the top portion 31 of the piston 3. Or, protruding portions may be provided to sandwich the oil guide surface 33 a from both sides in the circumferential direction of the piston 3, although not shown.

Or, as shown in FIG. 9B, the groove 32 and the swelling portion 34 may be combined. In an example shown in FIG. 9B, the groove 32 has a smaller depth than the groove 32 described in the above embodiment, and the groove bottom surface 32 a is continuous with a surface 34 a of the swelling portion 34 formed on the peripheral wall lower portion 30 b, to form an oil guide surface. By forming the groove 32 with a smaller depth, it is easy to ensure a wall thickness between the groove 32 and the ring groove 3 a.

A position of the oil jet 71 is not limited to the above described embodiment. For example, the oil jet 71 may be coupled to an oil passage which branches from the main gallery 66. This makes it possible to optimize the position of the oil jet 71 to inject the oil to the piston 3 while preventing the oil from contacting the connecting rod 25 or the crankshaft 26. In this case, the oil jet 71 is preferably positioned so as to inject the oil to the piston 3 from a timing corresponding to a location below a center of a stroke from the dead bottom center to the top dead center, when the piston 3 is moving upward from the dead bottom center to the top dead center.

Although in the present embodiment, the inline two-cylinder engine E has been described, for example, an engine of a single cylinder, an engine of three to six cylinders, a series engine, a horizontally opposed engine or a V-type engine may be used. Although in the present embodiment, the engine E is integral with the transmission, an engine which is not integral with a transmission, or an engine which is not provided with a transmission, may be used.

The cooling structure of the piston of the present embodiment is applicable to engines mounted in vehicles such as an all terrain vehicle, personal watercraft (PWC), etc., as well as a motorcycle. Of course, the cooling structure of the piston of the present invention is applicable to an engine mounted in a hybrid vehicle.

As this invention may be embodied in several forms without departing from the spirit of essential characteristics thereof, the present embodiments are therefore illustrative and not restrictive, since the scope of the invention is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims. 

What is claimed is:
 1. A cooling structure of a piston in an engine, in which oil is injected toward a top portion of the piston reciprocating within a cylinder of the engine to cool the top portion of the engine, comprising: an oil guide surface including at least a thick wall portion of a peripheral wall of the piston, which is provided with a ring groove and extends continuously from an inner surface of the peripheral wall of the piston to a reverse surface of the top portion of the piston.
 2. The cooling structure of the piston in the engine, according to claim 1, wherein protruding portions are provided to sandwich the oil guide surface from both sides in a circumferential direction of the piston.
 3. The cooling structure of the piston in the engine, according to claim 1, wherein the thick wall portion of the peripheral wall of the piston has a recess at an inner side thereof, and a bottom surface of the recess forms at least a portion of the oil guide surface.
 4. The cooling structure of the piston in the engine, according to claim 1, wherein a swelling portion is formed continuously with the thick wall portion of the peripheral wall of the piston, and the oil guide surface is formed to extend from the swelling portion to the thick wall portion.
 5. The cooling structure of the piston in the engine, according to claim 1, wherein the oil guide surface is formed at an exhaust side of the piston.
 6. The cooling structure of the piston in the engine, according to claim 1, comprising: an injection nozzle for injecting the oil to the piston, the injection nozzle being directly coupled to a main gallery of the oil inside of a crankcase.
 7. The cooling structure of the piston in the engine, according to claim 1, wherein an injection nozzle is positioned to inject the oil to the piston from a timing corresponding to a location below a center of a stroke from a bottom dead center to a top dead center, when the piston is moving upward from the bottom dead center toward the top dead center.
 8. The cooling structure of the piston in the engine, according to claim 1, wherein the oil guide surface is recessed relative to a portion of the peripheral wall of the piston which portion is adjacent to the oil guide surface in a circumferential direction of the piston.
 9. A cooling structure of a piston in an engine, in which oil is injected toward a top portion of the piston reciprocating within a cylinder of the engine to cool the top portion of the engine, comprising: an oil guide surface extending continuously from an inner surface of a peripheral wall of the piston to a reverse surface of the top portion of the piston, the oil guide surface being recessed relative to a portion of the peripheral wall of the piston which portion is adjacent to the oil guide surface in a circumferential direction of the piston.
 10. The cooling structure of the piston in the engine, according to claim 9, wherein an injection direction of the oil is substantially parallel to an axis of the piston, and the oil is injected from an opposite side of the piston with respect to a crankshaft of the engine.
 11. The cooling structure of the piston in the engine, according to claim 10, wherein in a state in which the piston is at top dead center, the oil guide surface is positioned so as not to overlap with a connecting rod of the engine when viewed from a direction of the axis of the piston.
 12. The cooling structure of the piston in the engine, according to claim 10, wherein the oil guide surface is positioned so as not to overlap with the crankshaft of the engine when viewed from a direction of the axis of the piston.
 13. The cooling structure of the piston in the engine, according to claim 12, wherein an injection position of the oil is set so as not to overlap with the crankshaft of the engine when viewed from the direction of the axis of the piston. 